I agree we have to be careful with fortified pellets. Not just the protein but the calcium too. One pellet that you can feed freely are Top's pellets, they're not fortified. If you can get Bonnie eating them you'll have a powerful tool for controlling her weight.
When I started on my journey I got a lot of help from a vet and between us we came to an easy diet - 25% seed, 25-35% fortified pellets (depending on activity levels) and the rest a mix of cooked grains and cooked and fresh veg. All this is measured by eye, a lot easier than measuring with a teaspoon or weighing.
We use the Top's pellets as a filler. If my flock is getting podgie they get more Top's and less of the other pellets.
We're at a stage now where they have 24/7 access to the Tops, seed goes in an hour before lights off and removed ½hour after lights on. Cooked grains and veg for breakfast, fresh veg chop at lunchtime and dinnertime. Between meals they get the fortified pellets - 1 tablespoon of a mix between 6 of them per day. When they moult they get some boiled egg white in their chop. An occasional slice of apple doesn't hurt (doesn't last long either
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It doesn't have to be scientific. It doesn't have to be accurate every day, it can be averaged weekly. Monitor the bird (can weigh them if they'll let you) and adjust the balance as needed.
Everybirdy is different. Watch the birdy.